Re: [PATCH] ALSA: seq: midi: wait for output buffer space on non-atomic delivery

[Junjie Cao]

On Tue, 01 Sep 2026 12:49:43 +0200, Takashi Iwai wrote:
> But, the problem is that you can't take long time in
> dump_midi() which is called from the sequencer event handler, per
> design; the atomic=false there doesn't mean that you are allowed to
> block for a too long time like 30 seconds.

Right, dropping this one.

> For working around this problem, we need a basic design change in ALSA
> sequencer core, I'm afraid.

Would you take an RFC along these lines, or do you have a different
shape in mind?

- event_input gets a way to say "port full": the core parks the event
  on a per-port FIFO instead of dropping it, copying direct/USRPTR
  payloads into the sender's pool cells (snd_seq_event_dup(),
  non-blocking).  While the FIFO is non-empty, further events for
  that port queue behind it.
- The kernel client signals readiness through a new snd_seq_kernel_*
  call and a work item re-offers the parked cells.  seq_midi would
  drive that from rawmidi's transmit ack, mirroring the input-side
  runtime->event hook.
- Backpressure stays at the sender's pool, the only place that sleeps
  today: a blocking writer with parked cells waits for pool space
  before its next direct dispatch, with ioctl_mutex dropped as
  snd_seq_cell_alloc() does; poll() already reports pool room.
  Nothing sleeps inside delivery.
- Client exit purges its parked cells from every port, as
  snd_seq_queue_client_leave() does for queues; port deletion frees
  the FIFO.

Open points: a SysEx bigger than the rawmidi buffer is consumed
partially, so the resume offset has to live in the parked entry or in
the driver; one event larger than the sender's pool cannot be parked
at all (the limit queued events already have); a broadcast is
duplicated per blocked destination.  Drivers that never report "full"
keep the current behaviour.